1. Field of the Invention
The present invention relates to a switching circuit and a small-size high-efficiency direct current-to-direct current (DC-DC) converter for portable devices including the same, and more particularly, to a switching circuit capable of reducing power consumption in a standby mode and conduction loss in a normal mode using a dynamic threshold-complementary metal oxide semiconductor (DT-CMOS), which is a dynamic threshold voltage device, as a switching device and a small-size high-efficiency DC-DC converter for portable devices including the same.
2. Discussion of Related Art
In general, portable devices use 3.6 V nickel cadmium (NiCad), lithium (Li)-ion, or Li-polymer batteries. However, internal circuits of portable devices use a voltage of 3.3 V or 2.5 V, and thus the battery voltage of 3.6 V needs to be reduced to 3.3 V or 2.5 V.
For this reason, power supplies of portable devices include DC-DC converters for converting voltage from a high level to a low level. To use batteries for longer, most portable devices employ a switch mode power supply (SMPS) DC-DC converter having high power conversion efficiency.
FIG. 1 illustrates a conventional SMPS DC-DC converter 100.
Referring to FIG. 1, in the conventional DC-DC converter 100, when CMOS transistors M11 and M12 of a switching circuit 110 alternately operate according to a pulse width modulation signal output from a pulse width modulator 130 and generate a square wave, an inductor L and a capacitor C filter the square wave and output DC voltage.
Here, the level of the DC voltage is determined by the pulse width modulation signal output from the pulse width modulator 130.
More specifically, when a difference amplifier 131 amplifies a difference between an output voltage Vout and a reference voltage and outputs the amplified difference, a comparator 133 compares the amplified difference with a triangle wave and determines a pulse width, and a square wave is generated according to the determined pulse width.
However, the DC-DC converter 100 inevitably has switching loss, conduction loss and control loss as described below.
First, the switching loss includes a loss caused by parasitic capacitance of the CMOS transistors M11 and M12, an overlap loss generated at a position at which voltage and current overlap during a switch transition, and so on.
Second, the conduction loss includes a loss caused by on-resistance when the CMOS transistors M11 and M12 are conducted, a loss caused by equivalent series resistance (ESR) of the inductor L and the capacitor C, and so on.
Third, the control loss includes a loss caused by pulse width modulation of the pulse width modulator 130.
While the switching loss and the control loss are uniformly generated regardless of output current, the conduction loss increases as the output current increases.
For this reason, to reduce the conduction loss, the CMOS transistors M11 and M12 used as a switching device are designed to have a large width. In this case, however, the chip size and power consumption increase.